Thanks Vitor, I don't, it was one of things i found with googling and double checked it.
Edit: i actually triple checked it, and if I turn the tab thing ON there, that function stops working, and shift tab properly autocompletes, so it's definitely something else on my system breaking tab in some subsection of apps
Edit edit: Finally. Either something modified my ~/Library/KeyBindings/DefaultKeyBinding.dict, or I did some time before and something else changed, but default Key Binding was trying to force "indent: " whatever that is (the default key binding dictionary is a bit of a black box tbh).
Thanks to everyone trying to help, I didn't really think it was an Alfred bug, I just wanted to double check that autocomplete used to work and I'm not misremembering